I am building a world that combines high fantasy and black powder, and would like some insight as to how warfare might be conducted in such a world.
In this world, an absence of magic for several millennia has lead to the development of technologies much like our own. While magic was not practiced, many magical texts and artifacts survived the years and serve mostly as a curiosity to the wealthy who could afford them. However, in the last hundred years, it is discovered that blood can be used to channel magical energies. While any blood will do, the blood of mythical creatures like manticores, chimeras and the like are much more potent for casting magic.
Now that magic has resurfaced, the rulers of the land have obviously began to pour resources into research and development of magical weaponry and mages for the use of warfare. While their numbers are limited compared to a standard soldier, these mages can cause tremendous damage on the battlefield.
Magic in this world works a lot like magic in a D&D setting, with better mages being able to cast better spells, and limited to a certain number of spells per day, with the exception of basic spells. In addition, mages require blood to cast these spells, from drops of blood for a simple frostbolt to a bottle for a fireball and beyond. As such, spellcasting is limited by mage ability and the amount of blood a nation can provide to its armies. It should also be noted that, where possible, mages might extract blood from fallen soldiers to fuel their casting.
Magic is not limited to evocation, but includes illusions, mind altering effects, and defensive wards, among other things.
The technology of this world is slightly inferior to civil war era technology. Most weapons are black powder, although there are experimental versions of repeating rifles and revolvers. The steam engine has just been developed and trains are limited both by technology and the lack of infrastructure to build networks of tracks. Armor is still useful, due to stronger steel and rare metals like adamantium, which can withstand gunshots. However, most likely full plate has been fazed out for armor that protected only the vitals.
While warfare during this era in our world consisted of large armies, tight formations, and an increase in unskilled combatants and in sieges, I imagine that this will not be the case with fireballs being thrown about.
With all that being said, How might magic affect renaissance era warfare?